Your little sister builds two tree houses and stretches a rope between them for sending messages to the other tree house. She hangs a mass of 1.4kg...

Your little sister builds two tree houses and stretches a rope between them for sending messages to the other tree house. She hangs a mass of 1.4kg on one end of the rope that passes over a pulley. The other end is tied to the other tree house. When she plucks the rope, a wave propagates at 18m/s. This is too slow, and she desires to increase the wave speed to 30m/s. She asks you, "What mass should I use?"